Any Layer HDI: Every Layer Interconnected by Microvia

What Makes It Different From Ordinary HDI

Conventional high density interconnect construction puts microvias on the outer layers, connecting the surface to the first inner layer or the first two. The core of the board is built with ordinary through vias, mechanically drilled, and the density benefit is limited to the surface layers where the fine pitch devices sit.

Any layer HDI removes that restriction. Microvias are used between every pair of adjacent layers, including between inner layers, so the whole stack is built up from laser drilled connections rather than from a drilled core. The result is that a signal can be routed from any layer to any other layer without a through hole occupying space all the way across the board.

The consequence for density is significant, because a through hole blocks routing on every layer it passes through, while a microvia blocks nothing except the layer pair it connects. On a design with many fine pitch devices, the through holes in the core become the limiting factor, and removing them is the entire point.

The consequence for manufacturing is equally significant, because the board is now built entirely by sequential lamination rather than by laminating a drilled core and adding two build up layers.

How the Stack Is Built

The process starts with a thin core, which is imaged and laminated. A dielectric layer is added on each side, laser drilled, plated and planarised. That cycle repeats until the required layer count is reached, with each repetition adding two layers to the board in the standard build up configuration.

A four layer any layer construction requires one build up cycle on each side. Eight layers requires three cycles on each side. Sixteen layers requires seven, and the process steps multiply accordingly: seven additional laser drilling operations, seven plating operations, seven lamination presses and seven inspections, all with the cumulative yield of the sequence applied.

That arithmetic is the core cost driver. Every additional cycle multiplies the number of process steps and compounds the yield, and it also adds to the accumulated registration error across the stack. This is why any layer HDI is a technology for the upper end of the density spectrum rather than a general purpose construction.

Via Structure Options

• Stacked microvias: each stage placed directly on top of the one below, forming a vertical column through several layer pairs. This gives the highest routing density, since the via occupies no lateral space, and it concentrates the mechanical stress at the interfaces between stages.
• Staggered microvias: each stage offset from the one beneath. Less dense, because the offset consumes lateral area, but the stress is distributed rather than concentrated, and the construction is generally more reliable under thermal cycling.
• Mixed constructions: any layer build up on the outer layers and a conventionally drilled core with through vias, which is the compromise used where full any layer construction is not required everywhere on the board.

The stacked against staggered decision is usually made on reliability grounds rather than on density alone. A stacked column of microvias experiences the accumulated thermal expansion of the entire height of the stack concentrated at the via interfaces, and the dielectric between stages is the weak point. Where the density budget allows staggering, it is usually the better engineering answer.

Design Rules Worth Holding To

• Aspect ratio of about 1:1 for each microvia, which constrains the dielectric thickness that can be used at each build up stage.
• Via in pad where the surface layer needs it, with filled and capped treatment so that the pad remains planar and does not wick solder during reflow.
• Stackup symmetry, since an asymmetric build up warps during the repeated lamination cycles.
• Impedance recalculation against the actual dielectric thickness at each layer pair, because the thin build up layers do not behave like a conventional stack.
• Via placement relative to the microvia stages, planning which layer transitions happen where, so that the build up sequence supports the routing rather than constraining it afterwards.

Because the stack is built up sequentially, the layer arrangement has to be planned with the build up sequence in mind. A routing change that moves a signal to a different layer can change how many build up cycles are required, which changes the cost of the whole board. That makes the stackup and the layout inseparable on an any layer design, and it is the reason the fabricator’s input is worth having before the routing is finalised.

Where It Is Used

• Smartphones and dense consumer devices, where the board area is fixed by the product and the component count keeps rising.
• Advanced system in package and package substrates, where the interconnect density requirement exceeds what conventional build up can provide.
• Wearables and medical devices, where both density and reliability matter and the volume is very constrained.
• High performance computing modules, where many high speed channels escape a large area array device in a small area.

Those applications share the characteristic that the routing requirement, not the circuit’s own complexity, drives the layer count. The HDI construction exists to resolve that, and any layer build up is the furthest extension of it.

Manufacturability Limits

• Dielectric thickness per stage: the microvia aspect ratio limit sets how thick each build up layer can be, which in turn limits the impedance range the stack can support.
• Registration accumulation: each lamination cycle adds alignment tolerance, so the total positional error grows with the number of stages. On a tall stack this is the factor that limits how far the build up can be pushed.
• Copper filling and planarisation: each stage has to be filled and flattened to a standard that allows the next layer to be laminated and drilled reliably.
• Cumulative yield: a process with a high yield per cycle still loses a meaningful fraction across seven or eight cycles, which is reflected in the price rather than in the specification.
• Panel handling: a thin build up stack is mechanically fragile during processing, which places demands on the handling equipment.

All of that means the capability question is not whether a fabricator can produce a sample, but whether the process holds across production volume with acceptable yield. The supporting fabrication capability, in terms of laser drilling, plating uniformity and lamination control, is what determines that, and it is worth asking for yield and cross section data on comparable constructions rather than accepting a capability statement.

Reliability

The reliability concerns on an any layer board are concentrated in the microvia structures.

• Thermal cycling: the interface between stacked microvia stages sees the accumulated expansion of the build up, and this is where interconnect failures appear. Thermal cycling testing with interconnect stress monitoring is the standard evaluation.
• Dielectric integrity between stages: the thin dielectric between a microvia and the stage beneath it is the mechanically weakest point in the structure, and a partial fill or a void in that region becomes a failure site.
• Via fill quality: voids in a filled via that sits in a component pad are both an assembly problem and a reliability problem.
• Staggered against stacked: staggering distributes the mechanical stress and is generally more robust, which is why high reliability applications tend to prefer it even at some cost in density.

Where the design is going into a product with a reliability standard, the microvia structures are part of the qualification evidence. This is where the fabricator’s quality system contributes directly, since the testing and the records are what demonstrate that the construction behaves as expected.

How It Compares With the Alternatives

• Conventional multilayer: mechanically drilled through vias, no laser drilling, lowest cost and most robust, but through holes occupy routing space on every layer.
• Build up HDI with a drilled core: microvias on the outer layers and through vias in the core. This resolves the routing density problem at the surface, where the fine pitch devices are, and is the right answer for most designs that need density.
• Any layer HDI: microvias everywhere. The highest density and the highest cost and process complexity.
• Alternative approaches: moving to a larger board, reducing the component count or accepting a less aggressive routing strategy can sometimes solve the same problem more cheaply than adding build up cycles.

The practical guidance is to use any layer construction only where the density requirement genuinely cannot be met otherwise. It is common to find that a design uses any layer build up for reasons of layout convenience rather than necessity, and each additional build up cycle is a significant cost that a redesign might avoid. That assessment is exactly what a design review with the fabricator is for, and it is cheaper than discovering the cost after the stackup is committed.

Frequently Asked Questions

What is any layer HDI? High density interconnect construction in which microvias are used between every pair of adjacent layers, including inner layers, rather than only on the outer layers. The whole stack is built up by sequential lamination.

Why does it cost more than conventional HDI? Because each build up stage adds a laser drilling operation, a plating operation, a lamination press and an inspection, and the yields of those steps compound. A sixteen layer any layer board requires seven build up cycles per side.

Should microvias be stacked or staggered? Stacked gives the highest density. Staggered distributes the mechanical stress and is generally more reliable under thermal cycling. The choice should be made on the reliability requirement where the density budget allows it.

What limits how many layers can be built this way? The accumulation of registration error across the lamination cycles and the cumulative yield. Both get harder as the stack grows.

When is it the right choice? When the routing density requirement cannot be met by a build up construction with a drilled core, typically on very dense fine pitch designs where through holes in the core are the limiting factor.

Summary

Any layer HDI extends high density interconnect construction to the whole stack, using laser drilled microvias between every layer pair instead of a mechanically drilled core with through vias. The benefit is routing density, because a microvia occupies only the layer pair it connects, while a through hole blocks every layer it passes through.

The cost is process complexity. The board is built entirely by sequential lamination, with each build up cycle adding laser drilling, plating, lamination and inspection, and the yields of those steps compounding. Sixteen layers requires seven cycles per side, which is where the price comes from.

The engineering decisions follow from that. Plan the stackup with the build up sequence in mind, keep each microvia at an aspect ratio near 1:1, choose stacked or staggered microvias on reliability grounds rather than by default, and confirm with the fabricator that the required density cannot be met by a cheaper construction. Where the density requirement is real, any layer build up is the only way to meet it. Where it is not, the additional build up cycles are cost without benefit.